Is it fine to use a common mode choke as a 1:1 isolation transformer https://www.digikey.com/products/en?keywords=744822233 to power a small 5v signal generating circuit? The only issues seem to be saturating the transformer.

 

The only issues seem to be saturating the transformer.

Yes, you need to know that to determine the minimum operating frequency.

You can measure that by applying a small DC voltage and observing the current waveform.
It will be a linear ramp until the core starts to saturate.
Then the current will start to increase more rapidly.

 

Maybe I mis-understand, but I don't think the choke will provide isolation.

 

Maybe I mis-understand, but I don't think the choke will provide isolation.

A common-mode choke has two identical, isolated windings.

 

An RMS DMM should be fine for current measurments?

No. You need an oscilloscope to see the rapid change in current.

 

A common-mode choke has two identical, isolated windings.

But I don't think it will make a good transformer. I believe that the core is designed to particularly lossy to absorb the interference and this is not what you would want for a transformer.

 

You better look for coupled inductors, those are meant for flybacks and isolated buck converters etc. https://www.digikey.com/en/product-highlight/w/wurth-electronics/coupled-inductors

 

In a very few instances a dual common mode choke has been used as an isolation transformer, BUT not in a common-mode-choke connection arrangement. And at best it is not a very good transformer, since it is not at all designed for that application.

 

I believe that the core is designed to particularly lossy to absorb the interference

Net necessarily.
The choke is designed to generate a high impedance to block common-mode currents, not absorb them, so I don't see how a lossy core would help that.

 

Doesent oscilloscope waveform depicts frequency response while current draw measured with dmm indicate saturation?

An oscilloscope displays any change in voltage with time, not just frequency response.
By measuring the current across a small resistor in series with the inductor, you can view the rapid current ramp change with time of the inductor.

A DMM measures DC or AC voltage.
It cannot measure a rapid ramp change in voltage with time.

For example, applying 1V to a 3mH inductor will give a current ramp of 1/3A per millisecond (LTspice simulation below).
That can't be measured with a DMM.
(The simulation inductor model does not show the saturation the would cause the slope to curve up as it saturates).

 

Net necessarily.
The choke is designed to generate a high impedance to block common-mode currents, not absorb them, so I don't see how a lossy core would help that.

My understanding was that if the common mode choke is perfect, then the RF energy the circuit generates gets reflected back and though that cures high conduted emisions you might end up increasing radiated emissions.

 

My understanding was that if the common mode choke is perfect, then the RF energy the circuit generates gets reflected back and though that cures high conduted emisions you might end up increasing radiated emissions.

Correct, just about any inductive suppression device has lossy materials (like ferrite) that are designed to convert suppressed energy to heat.

 

My understanding was that if the common mode choke is perfect, then the RF energy the circuit generates gets reflected back and though that cures high conduted emisions you might end up increasing radiated emissions.

Okay.
But the only way the choke can absorb the RF is to let it through the choke, so then it's not doing its job of blocking the conducted radiation.
I would think you want the choke to do the best job it can to block the conducted emissions, and use other means to suppress the radiated emissions (such as shielding).

Note that the reason for a lossy ferrite, such as in ferrite beads, is to minimize the resonance of the inductance with any circuit capacitance, not to absorb the noise.

 

Rule #1 in emission control.

You can't destroy energy, only convert or divert it.

Notice the very high R value for a inductor.
https://www.murata.com/en-us/products/emiconfun/emc/2011/06/14/en-20110614-p1

Fig. 3 shows an example of the impedance frequency characteristics of a chip ferrite bead. The basic principle involved is as follows: The impedance increases in proportion as the frequency rises, as in the case of inductors, so by connecting these beads in series in a circuit, they function as a low-pass filter. With regular inductors, the main characteristic among the impedance (Z) values is the reactance component (X). On the other hand, since chip ferrite beads use ferrite materials with a high loss in the high frequencies, the main characteristic in the high-frequency range is the resistance component (R). The reactance component is not accompanied by loss, but the resistance component is. This means that, compared with regular inductors, chip ferrite beads have better properties for absorbing noise energy, providing a higher noise-suppression effect.

 

But the only way the choke can absorb the RF is to let it through the choke, so then it's not doing its job of blocking the conducted radiation.

Not the only way, the lossy nature of the choke lets the RF energy convert to heat, acting like a termination at those frequencies, instead of letting it get reflected back to where it came from and being radiated further from there.

The termination impedance is of course not the proper impedance at all frequencies of interest, but usually it is good enough to extract significant amount of energy and let you fit into the EMC limits.

 

Note that the reason for a lossy ferrite, such as in ferrite beads, is to minimize the resonance of the inductance with any circuit capacitance, not to absorb the noise.

Not the only reason. You could use the same inductance and put in enough capacitance, so that the other parasitics dampen that resonance well enough. But it also serves to convert energy with frequencies above the passband to heat instead of being stored in the magnetic field and recirculated.
But now that I look at it that looks like two ways of describing the same thing, dampening resonance while also dampening any other energies that don´t necesarily resonate with the caps.

Imagine a ferrite core put over a cable, main purpose is energy absorption and not added inductance.

 

Please notice that the original post, #1, was asking if a dual winding common mode choke could be utilized as a 1:1 isolation transformer. And the answer to that is "Yes BUT" and the reality is that it has been done and it has worked but it is not always going to be a satisfactory transformer, except for a few applications. The original question has been quite completely ignored, I am not certain just why, but it was.

 

Can ferrite beads actually worsen noise instead of attenuate it?

If used correctly, no.

 

Please notice that the original post, #1, was asking if a dual winding common mode choke could be utilized as a 1:1 isolation transformer. And the answer to that is "Yes BUT" and the reality is that it has been done and it has worked but it is not always going to be a satisfactory transformer, except for a few applications. The original question has been quite completely ignored, I am not certain just why, but it was.

There was a comment made that unfortunately side-tracked the good information from your original post about possible issues when using one as a transformer.